1. Field of The Invention
This invention relates to compound bows, particularly to compound bows with high peak draw forces followed by an extreme reduction in draw force at extension, and apparatus and method therefor.
2. Description of Related Art
A compound bow does not behave like a simple spring when it is drawn. A compound bow requires a great deal of force to pull it at first; the required force then "lets off" after some peak to a more reasonable level. The bow is "compound" in that it has a non-linear force-draw characteristic.
An early American patent on a compound bow was awarded to Holless W. Allen, of Kansas City, Mo., as U.S. Pat. No. 3,486,495, issued in 1969. Mr. Allen's bow represents a considerable departure from previous designs, and the principles he originated have been refined by many others. Modern manufacturers of compound bows, making bows essentially based on Mr. Allen's design, advertise let-offs of up to 65% on bows with a peak draw force around 36 kilograms (80 pounds). Mr. Allen's design uses eccentric wheels at the bow limb ends, with crossing wires between the limb ends to synchronize limb movement. The eccentric wheels provide a variable mechanical advantage as the bow is drawn. The Allen design has been a great commercial success and represents, in its descendent designs, the only type of compound bow generally available on the market. The design has one major weakness: the amount of let-off is fundamentally limited by the design. Several other features also represent weaknesses in the design. For example, the aesthetics of eccentric wheels and multiple crossing cables are poor, the bow is difficult to adjust so that equal forces can be obtained from each limb; and the balance of the bow is generally not at the handle but at a point toward the user. Furthermore, a high proportion of the total weight is located at the limb ends, where the potential for deleterious dynamic effects is greatest. Balance is an important consideration because of its effect on the user's ability to control the arrow release. A center of gravity toward the user will cause the bow to drop slightly tip on release as it pivots around the user's hand, thus causing porpoising of the arrow in flight. Any departure from exact balance of the limb ends will have the same effect. A center of balance forward of the user's hand is generally preferred.
The eccentric-wheel approach is now widely used, but it is not the only method for achieving a compound bow. A second approach is based on the fact that when rotating a cam or lever against a spring, the force imparted by the spring may be divided into a component causing a rotational torque and a second component causing compression of the cam or lever. When the contact point of the cain or lever is in a direct line between the spring and the pivot point of the cam or lever, there is no torque component at all. The force from the spring is completely expended in compressing the cam or lever. If this is taken as the zero angle, then the component of force causing torque as the cam or lever is rotated may be calculated by multiplying the force from the spring by the sine of the angle of rotation. The peak torque component will be at ninety degrees. It must be remembered, however, that a spring generally compresses linearly, causing a variable force as a cam or lever is rotated against it. This principle applied to a compound bow was first patented in the U.S. by George C. Smith, of McKinleyville, Calif., as U.S. Pat. No. 3,812,835, issued in 1974.
Mr. Smith's bow incorporates rotatable limbs pivoted on a handle assembly and bearing against leaf springs mounted at their base on the handle assembly. As the a rotatable limbs pivot on the handle assembly the force from the leaf springs will increase as they are deflected, but the resultant torque will decrease as the angle between spring contact and pivot point is reduced. The mechanical advantage of the bowstring on the arrow it propels also varies as the bow is drawn, making analysis somewhat more difficult, but the end result of Smith's design is a force-draw function with a peak and a subsequent reduction to zero if desired. There is, however, a very fundamental problem with Mr. Smith's novel design. Mr. Smith is aware that any deviation in the force from the springs or geometry of the mechanism will cause one limb to bend more than the other, so synchronizing cables are necessarily incorporated between the two limbs. Unfortunately, however, the attachment points of the coordinating cables depicted in Mr. Smith's patent are clearly at different radii from the pivot point of the rotatable limbs, thus causing the amount of cable "paid out" and "drawn in" during rotation to be quite different for the two cable attachment points. Since the inner attachment point of one limb is directly cabled to the outer attachment point of the other, and vice versa, an equal amount of cable must be paid out and drawn in from both the inner and outer attachment points to allow rotation. When attachment points are at different radii, as in Mr. Smith's configuration, no limb rotation at all is mechanically possible.
A second approach to a compound bow using a spring and rotating lever or cam was made by Robert M. Van House, of Dayton, Ohio, U.S. Pat. No. 4,041,927, issued in 1977. Mr. Van House's design has several distinctive features. Only one spring is used, in this case a piston compressing air or a rubber shear spring, thus assuring that the force exerted on both rotatable limbs is equal. Limb synchronization is accomplished with a single cable routed over forward and after portions of pulley sections attached to the upper and lower rotatable limbs of the bow respectively. Linkages to the spring are adjustable to accommodate different draw lengths, and the spring itself is adjustable by varying the air pressure therein. By placing the spring forward of the handle good balance can be achieved. But the design has a significant weakness in that it does not lend itself to the production of high power bows. If, for example, the object is to design a compound bow with a peak of 36 Kilograms (80 pounds) of draw force, the air spring of Mr. Van House's bow must exert a force of roughly 775 Kilograms (1705 pounds), assuming a bow with a 122 cm. (48 inch) string proportioned as shown in Van House's FIG. 1. This would require a piston air pressure around 60.8 Kg/cm.sup.2 (865 lbs/in.sup.2), which is not easy to work with in practice. The structure of the linkages and bearing points shown in Van House's FIG. 1 would also need strengthening to withstand the forces applied.
U.S. Pat. No. 4,756,295 issued to Matthew P. Guzzetta of Spring Valley, Calif., in 1988, is very similar in concept to the Van House patent cited above, and suffers from the same defect. Mr. Guzzetta's design is also conceptually attractive but not practical when applied to a high powered bow.
A large amount of energy must be stored in a high powered bow when it is drawn or there cannot be a large amount of energy to ultimately propel the arrow. When an attempt is made to store that energy in a small place, such as in an air or compression spring, then that energy becomes concentrated and high resultant forces develop that must be contained. Friction at bearing surfaces also increases. The primary advantage of the compound bow invented by Allen and refined by many others is that energy storage takes place in the limbs as in a conventional bow, thus distributing the energy over a larger volume.